Sensors and systems incorporating them generally put out voltages, frequency shifts or some other signals, rather than the amount of the physical, chemical or biological entity that is of interest to users. In order to recover the quantitative values of interest, for example, the concentration of a poisonous gas, it is necessary to employ a sensor calibration curve, which relates the signals that are not of direct utility to the amount (with proper units) of the entity being measured (the measurand). The measurement of a calibration curve requires producing and independently measuring the amount of the measurand, exposing the sensor or sensor system to that amount and reading the signal produced by the sensor or system. This is usually done at several values of the measurand in order to fit a mathematical curve to the data to produce the calibration curve.
For a variety of reasons, the performance of a sensor system can deviate from the calibration curve with age and the amount of use. It is necessary to be able to verify the performance of the sensor or system against the original calibration curve at various times during the lifetime of the sensor or system. This is especially true for instruments used by first responders, which may not be turned on for long intervals. However, it also applies to devices for screening of air passengers, which are used daily, and for process control, which requires continuous use for long periods. In all events, returning sensor systems to a laboratory for calibration checks is unacceptable because of cost reasons. It is highly desirable to be able to do performance or recalibrations in the field or factory.
In typical calibration, the first question is whether the instrument is working at all. This can be answered by turning on the device, and then seeing if the output has the correct value. The next question is whether or not the instrument responds reasonably to the chemical of interest. This can be answered by producing a vapor or mist of that chemical at the intake to the instrument to see if the output rises and falls with the appropriate time scale. The remaining question is whether or not the instrument is correct quantitatively. To answer this question, it is necessary to do a recalibration at a minimum of one point. As with any calibration, two numbers are needed, namely the value of the measurand and the amount of signal elicited by that amount. One way to obtain the amount of the measurand quantitatively is to produce and measure it independently, as noted above for obtaining the original calibration curve. This almost always requires returning the instrument to a calibration laboratory.